Conventionally, ethylene and propylene are produced via steam cracking of paraffinic feedstocks including ethane, propane, naphtha and hydrowax. An alternative route to ethylene and propylene is an oxygenate-to-olefin (OTO) process. Interest in OTO processes for producing ethylene and propylene is growing in view of the increasing availability of natural gas. Methane in the natural gas can be converted into for instance methanol or dimethylether (DME), both of which are suitable feedstocks for an OTO process.
In an OTO process, an oxygenate such as methanol or dimethylether is provided to a reaction zone of a reactor comprising a suitable conversion catalyst and converted into ethylene and propylene. In addition to the desired ethylene and propylene, a substantial part of the oxygenate such as methanol is converted into higher hydrocarbons including C4+ olefins, paraffins and carbonaceous deposits on the catalyst. The effluent from the reactor comprising the olefins, any unreacted oxygenates such as methanol and dimethylether and other reaction products such as water may then be treated to provide separate component streams. Unreacted oxygenates can be separated from the reaction effluent, for instance by contacting with a cooled aqueous stream in a quench tower.
In order to increase the ethylene and propylene yield of the process, the C4+ olefins may be recycled to the reaction zone or alternatively further cracked in a dedicated olefin cracking zone to produce further ethylene and propylene.
Due to the high temperatures in the reaction zone and the acidity of the catalyst, a portion of the oxygenates such as methanol may unavoidably decompose thermally or catalytically into oxides of carbon, i.e. carbon monoxide and carbon dioxide in the gaseous form. The carbonaceous deposits on the catalyst can be removed by the periodic regeneration of the catalyst by heating it with an oxidising gas such as oxygen, in order to burn off the deposits.
Carbon dioxide generated during the OTO process is an acid gas which is thus present in the effluent from the reactor. In order to prevent contamination of the olefinic product and problems associated with the formation of solid carbon dioxide during the separation of the olefinic product into olefinic component streams, which may be carried out at cryogenic temperatures, carbon dioxide should be removed from the reaction effluent and from the gaseous effluent from the quench tower before separation into olefinic component streams. This is typically done by washing the gaseous effluent with a caustic solution in a caustic tower.
Carbonyl compounds, such as aldehydes and ketones, in particular acetaldehyde, are commonly generated by the catalyst in side reactions and are also found in the effluent from the reactor. Carbonyl compounds may build up in the caustic solution used to remove carbon dioxide and other acid gases. The basic components of the caustic solution, such as hydroxide ions, can catalyse the aldol condensation and subsequent dehydration reactions of particularly acetaldehyde to form unsaturated aldehydes such as acrolein, especially at higher pH, such as a pH above 9. Unsaturated aldehydes may polymerise when allowed to accumulate in the caustic solution and if the aldol condensation reaction is left unchecked, a viscous oily polymer can be formed, known as ‘red oil’, which is insoluble in the caustic solution and can deposit on equipment internals, causing fouling and leading to maloperation of the caustic tower containing the caustic solution.
WO 2007/111744 discloses a process for oxygenate conversion to olefins with enhanced carbonyl recovery. A recycle or circulated water stream is treated with a sulphite-containing material in order to form a treated water stream with an appropriately reduced or minimised carbonyl, in particular aldehyde, content. The sulphite-containing material is added to the oxygenate absorber zone. The oxygenate-rich water stream containing unreacted sulphite and bisulphite addition compounds produced in the oxygenate absorber zone is passed to an oxygenate stripper zone to be separated into an oxygenate-containing stream and a recycle water stream. The oxygenate-containing stream can be returned to the oxygenate conversion reactor. The recycle water stream can be passed to a wash water stripper to recover oxygenates and provide a bottoms water stream comprising unreacted sulphite and bisulphite addition compounds which can be passed to the effluent treatment zone for the treatment of the reactor section effluent. The recycle water stream can also be passed to the oxygenate absorber zone for the treatment of the compressed oxygenate conversion effluent stream.
A disadvantage of the process of WO2007/11174 is, however, that a stream comprising unreacted sulphite and bisulphate addition compounds is treated in the effluent treatment system of the OTO process. This may result in undesired release of acetaldehyde from its addition compound, since the formation of formaldehyde addition products is favoured. Moreover, a sulphur-containing waste stream will be formed in an olefins-to-oxygenate process that normally produces a sulphur-free effluent.